An increasing demand for electronic equipment that is smaller, lighter, and more compact has resulted in a concomitant demand for semiconductor packages that have smaller outlines and mounting “footprints.” One response to this demand has been the development of the “flip-chip” method of attachment and connection of semiconductor chips to substrates. Sometimes referred to as the “Controlled Collapse Chip Connection,” or “C4,” method, the technique involves forming balls of a conductive metal (e.g., solder or gold) on input/output connection pads on the active surface of the chip, then inverting, or “flipping” the chip upside-down, and “reflowing” the conductive balls (i.e., heating them to the melting point) to fuse them to corresponding connection pads on a substrate.
Another response has been the development of ball grid array (BGA) semiconductor packages that “surface mount” and electrically connect to an associated substrate (e.g., a printed circuit board (PCB)), with a plurality of solder balls in a method, sometimes referred to as the “C5” method, that is analogous to the flip-chip method described above for mounting and connecting dies.
In both the C4 die and C5 package mounting and connection methods, a plurality of solder balls are attached to respective solder ball mounting lands, or pads, defined on a surface of the die or package. The solder ball mounting pad can, but need not be, defined by an opening in an insulative layer or mask called a “passivation layer” in the case of a semiconductor die, or a “solder mask” in the case of a BGA package, as described below.
FIG. 1 illustrates a schematic cross-sectional view of a substrate 10 having a solder-mask-defined (“SMD”) solder ball mounting pad 12 formed thereon in accordance with the prior art. The substrate 10 can comprise a sheet 14 of an insulative material, such as fiberglass, polyimide tape, or ceramic. Alternatively, it may comprise a semiconductor chip or die.
The mounting pad 12 typically comprises a layer of metal (e.g., copper, aluminum, gold, silver, nickel, tin, platinum, or a combination of the foregoing) that has been laminated and/or plated on a surface of the sheet 14, and then patterned using known photolithography techniques into a contact pad structure 16, which may include one or more circuit traces (not shown) radiating outward from it.
An insulative mask 20, referred to as a passivation layer in the case of a semiconductor die, or a solder mask in the case of a BGA package, is formed over the metal layer, including the contact pad 16. The insulative mask 20 may comprise an acrylic or a polyimide plastic, or alternatively an epoxy resin, that is silk screened or photo-deposited on the sheet 14. An opening 22 is formed in the insulative mask 20 to expose a central portion 28 of the contact pad 16, and a solder ball 30 is attached to the central portion 28 of the contact pad 16 thus exposed. Since the mask 20 prevents the solder of the solder ball 30 from attaching to any portion of the pad 16 other than the central portion 28 that is exposed through the opening 22, the pad 16 is referred to a solder-mask-defined or SMD-type of solder ball mounting pad, as above.
The SMD pad shown in FIG. 1 provides good “end-of-line” (i.e., at the end of the semiconductor package fabrication line) ball 30 shear resistance because the insulative mask 20 overlaps the entire peripheral edge of the pad 16, and hence, resists ripping of the pad from the sheet 14 when mechanical forces act on the solder ball 30 attached thereto. The SMD pad also affords relatively stable control of the “x-y” positional tolerances of the solder ball 30 (i.e., control of the lateral position of the solder ball 30 on the surface of the sheet 14).